(1) Field of the Invention
The instant invention relates to fiber optic devices for distributed sensing applications, and more particularly to a fiber optic sensing device using a series of one or more ultra-weak, terahertz-range reflectors, reflector cavities, or reflector structures to perform interferometric measurements.
(2) Description of Related Art
Waveguide-based sensing devices have increasingly demonstrated their utility in recent years, leading to their expanding adoption in areas previously dominated by more traditional sensing methodologies. This growth is due in large part to the several distinct advantages waveguide-based sensors have over other, earlier methods. These advantages include the ease with which such devices can be multiplexed and simultaneously interrogated along a single waveguide structure, thereby allowing for distributed sensing over a substantial distance with high spatial and temporal resolution.
Fabry-Perot interferometers, Bragg gratings, and other periodic reflector structures are mature sensing techniques that have been widely used for strain, stress, pressure, and temperature measurement. Through the integration of these structures into a variety of waveguides, the utility of these technologies has been successfully demonstrated over a broad set of frequency ranges. In the optics domain, incident frequencies in the hundreds of terahertz are routinely used to interrogate periodic fiber reflector structures. By resolving shifts in the reflected spectra, subtle changes in the parameters of interest can be precisely measured. Similar utility has been demonstrated in the microwave domain (several gigahertz) through the successful implementation of coaxial cable Bragg gratings (CCBGs) fabricated by introducing physical discontinuities in the cable structure at the centimeter scale.
Fiber reflector structures and CCBGs have demonstrated their utility for large scale, multiplexed sensing applications. However, these techniques have distinct limitations. For example, the large frequency ranges necessary for interrogation in the optical domain require broadband swept frequency lasers, which are very expensive, or a combination of a broadband light source and optical spectrum analyzer, with broad ranges (tens of nm, or a few terahertz, at a wavelength of around 1550 nm). In the microwave domain, the long pitch-length of coaxial cable gratings limits spatial resolution (tens of cm) for sensing applications.